Involvement of Hedgehog Signaling and Calcium Channels During Germ Cell Migration in Drosophila melanogaster

Abstract

During the embryonic development of Drosophila melanogaster the primordial germ cells (PGCs) must migrate towards and coalesce with the somatic gonadal precursor cells (SGPs) to form the primitive gonad. While the precise mechanisms involved in PGC migration have yet to be fully elucidated, our previous data has suggested that the signaling ligand Hedgehog (Hh) emanating from the SGPs functions as a germ cell attractant that is potentiated by HMG-CoA reductase (Hmgcr). In this study we extend this model and offer substantial support for the claim that Hh functions as a PGC attractant. Using loss- and gain-of-function experiments, we demonstrate that the hh-pathway gene shifted is required for PGC migration and the transmission of the attractant. Furthermore, we show that Hmgcr functions as the key specificity factor that augments the Hedgehog ligand and enables it to function as a long-range chemoattractant. By tracking the transmission of Hh-GFP fusion protein, we offer direct evidence that Hmgcr potentiates Hh release and/or transport and that PGCs interact with the Hh ligand. We also directly compare the relative strengths of differentially modified forms of Hh ligand to influence PGC migration, and these data have led to novel insights into the possible role of apical vs. basolateral transport of Hh from the Hh-producing cells. Taken together, our data substantiate the novel claim that Hh can function not only as a morphogen but also as a chemoattractant for the migrating germ cells. In the next part of this study, we examine cell autonomous factors that likely enable PGCs to transduce reception of the Hh signal into directed motility. To this end we perform a functional analysis of the TRP calcium channels TRPA1 and Painless, both of which have previously been implicated in Hh-dependent nociception. We find that the activities of both channels are required for proper PGC migration, which indicates that calcium flux likely plays a critical role in enabling PGCs to undergo directed motility in response to the Hh attractant.